I convert my old world from 1.6.4 and I am in process restore lost mods blocks in my buildings.
And I found that Red Rock Bricks Slab missing in block list (bricks and bricks stair listed).
Used version: ExtrabiomesXL-universal-1.7.10-3.16.a18.jar

Shortly after I found recent commit bfd192e related to slabs and build jar for current mod sources. Problem fixed in it! So issue can be considered fixed in next beta release! )